Analog-to-digital converters are in widespread use today in electronics for consumers, industrial applications, etc. Typically, analog-to-digital converters include circuitry for receiving an analog input signal and outputting a digital value proportional to the analog input signal. This digital output value is typically in the form of either a parallel word or a serial digital bit string. There are many types of analog-to-digital conversion schemes such as voltage-to-frequency conversion, charge redistribution, delta modulation, as well as others. Typically, each of these conversion schemes has its advantages and disadvantages. One type of analog-to-digital converter that has seen increasing use is the switched capacitor sigma-delta converter.
As many analog-to-digital converters, the switched capacitor sigma-delta converter uses a digital-to-analog converter DAC in a feedback loop as shown in FIG. 1 and cannot be more linear than the digital-to-analog converter. An input signal U is fed to a Loop Filter. The output signal Y of the Loop Filter is forwarded to a Quantizer which provides the output bitstream V. This bitstream V is fed back to the DAC whose output is fed back to the Loop Filter. Therefore a very linear digital-to-analog converter is required in order to achieve a perfectly linear analog-to-digital conversion. However a high resolution is not required for the digital-to-analog converter used in the feedback loop of a sigma-delta converter: The digital-to-analog resolution can be exchanged with the over-sampling ratio at the cost of a longer conversion time.
A two-level digital-to-analog converter is inherently linear and thus is not the limiting factor for the accuracy of a sigma-delta converter. Therefore it is the standard approach in a sigma-delta analog-to-digital converter. Such A/D converters are for example disclosed in the article “A 192 ks/s Sigma-Delta ADC with integrated DecimationFilters Providing −97.4 dB THD” by Mark A. Alexander, Hessam Mohajeri, and Justin O. Prayogo in IEEE International Solid State Circuit Conference 37 (1994) February, New York, US, and “Theory and Practical Implementation of a fifth-Order Sigma-Delta A/D Converter” by R. W. Adams, P. F. Ferguson, A. Ganesan, S. Vincelette, A. Volpe, and R. Libert in AES Journal of the Audio Engineering Society 39 (1991) July/August, No. 7/8., New York, US. A five level feedback digital-to-analog converter is also known from U.S. Pat. No. 7,102,558 assigned to Applicant which is hereby incorporated by reference.
In sigma delta converters, capacitive charge transfer DACs are often used to realize the feedback of the modulator if the modulator is made of switched capacitors. Multi-bit architectures have nice advantages including less quantization noise, more stability, less sensitivity to idle tones as well as better distortion behavior. Since the DAC output resides at the input of the modulator, the inaccuracies of the DAC are directly transmitted to the signal and are difficult to compensate for. Therefore, it is critical to be able to realize linear DACs with as many levels as possible (making a multi level flash ADC is easier since in a sigma delta modulator, it does not require as much accuracy as the DAC as it stands at the end of the signal chain). Multi-level DAC with more than 5 levels require multiple capacitors and dynamic element matching to be able to transfer the signals in two phases (most of the sigma delta modulators based on switched capacitors have two phases one for sampling signals one for transferring signals to the next stage). These multi-level DACs are typically realized as charge transfer DAC. In these type of DACs, each output level is defined by a different amount of electrical charge transferred to the output of the DAC. Thus, a charge transfer DAC is transferring charges and therefore operates differently than a voltage or current DAC.
However, multi-level DAC with more than five levels require multiple capacitors and dynamic element matching to be able to transfer the signals in two phases (most of the sigma delta modulators based on switched capacitors have two phases: one for sampling signals and one for transferring signals to the next stage).